It would make sense if the S9 buffered writes internally. They're certainly using slower cheaper flash media and people would be thrilled at how they were able to transfer their handful of MP3 files instantly even though the S9 is still secretly writing to the internal media.

The buffer (if it exists) seems to be about 300MB. Once you write that much, the writes start blocking for a very long time. Even if you write slightly less than that, a sync or a umount will still take a long time since the device has to confirm the data has been written correctly.